Lateral flow assays are devices that are used to detect the presence (or absence) of a target analyte in a sample fluid without the need for specialized equipment. The lateral flow assays are widely used for medical diagnostics for point of care testing, home testing, or laboratory use.
A lateral flow assay typically includes a series of capillary pads for transporting fluid. A sandwich assay format may be used for detecting analytes that have at least two binding sites to bind to an antibody. A sample pad is used to receive a quantity of fluid (referred to as the sample fluid) and transport the sample fluid to an adjacent conjugate pad. The conjugate pad contains a solubilized antibody labeled with a detector such as colloidal gold nanoparticles. The antibody is specific to a certain analyte which is the target of interest in the sample fluid. As the sample fluid flows through the conjugate pad, the analyte (if any) in the sample fluid binds with the labeled antibody on the conjugate pad and forms an immunocomplex.
The immunocomplex then flows from the conjugate pad into an adjacent membrane (or membrane pad). The membrane has a test area, or test line, that contains an immobilized unlabeled antibody. As the immunocomplex moves over the test area, the immunocomplex binds with the immobilized antibody on the test area, resulting in a colored test line. When the sample fluid does not include the target analyte, no immunocomplex is formed on the conjugate pad and no immunocomplex binds with the immobilized antibody on the test area. As a result, the test line does not change color.
A lateral flow assay may also include a control line in the membrane. In a sandwich assay format, the control line may contain an immobilized antibody that binds to the free antibodies labeled with the detector resulting in a colored control line, which confirms that the test has operated correctly regardless of whether or not the target analyte has been present in the sample.
A competitive assay format may be used for detecting analytes that cannot simultaneously bind to two antibodies. The sample pad and the conjugate pad in a competitive assay format are similar to the sample pad and the conjugate pad in the sandwich assay format. In the competitive assay format, the test line contains immobilized analyte molecules.
If the sample liquid does not contain the analyte, the labeled antibody flows from the conjugate pad into the test line and binds to the analyte at the test line, resulting in a colored test line that indicates the lack of the target analyte in the sample liquid. If, on the other hand, the target analyte is present in the sample liquid, the analyte binds to the labeled antibodies on the conjugate pad and prevents the labeled antibody to bind to the analyte at the test line, resulting in the lack of color on the test line. In a competitive assay format, the control line may contain an immobilized analyte that binds to the free antibodies labeled with the detector resulting in a colored control line, which confirms that the test has operated correctly regardless of whether or not the target analyte has been present in the sample.